Paper, in the form of “boxboard” or “paperboard,” has been widely used for such disposable items as drinking cups and containers for liquids, such as milk and fruit juices. For these applications, paper has the advantages of low toxicity, low cost, printability, biodegradability, and the ease with which it may be formed into the required shapes. However, untreated paper is not suitable for the aforementioned applications, because it is permeable to water and other aqueous and non-aqueous fluids.
It is well known in the art to coat materials and substrates with a fluorochemical coating, in order to impart oil and grease resistance to the materials and substrates. For example, Schwartz, “Oil Resistance Utilizing Fluorochemicals,” TAPPI, Seminar Notes, 74, 71–75 (1987) discloses the use of commercially available FDA-cleared fluorochemicals to impart resistance to low surface tension fluids on various substrates. U.S. Pat. No. 4,426,466 discloses treatment compositions containing fluorochemical carboxylic acid and epoxidic cationic resin to impart oil and water repellency to cellulosic materials. U.S. Pat. No. 4,529,658 discloses fluorochemical copolymers useful for imparting oil and water repellency to cellulosic and textile materials. U.S. Pat. No. 5,370,919 discloses fluorochemical compositions for imparting oil and water repellency to various substrates.
Two well-known commercially available, fluorochemical-containing products for imparting oil and grease resistance to a substrate are Scotchgard® and Scotchban®, both of which are manufactured by the Minnesota Mining and Manufacturing Co. (3M). Scotchgard®, Scotchban® and other similar commercially-available products have heretofore been applied to a wide variety of goods for purposes of oil and grease resistance, including carpet, clothing, fabric, home furnishings, leather, outerwear, upholstery, automotive fabrics and carpets, film and photographs.
With regard to providing oil and grease resistance to paper, 3M manufactures a product similar to Scotchguard®, under the brand name Scotchban®. Scotchban® is a water soluble fluorochemical sizing agent that imparts grease and oil resistance to paper, paperboard and pigmented coatings. It is believed to presently conform to Food and Drug Administration (FDA) regulations for use in direct contact with non-alcoholic foods under certain prescribed conditions of use. Scotchgard®, Scotchban® and related products have heretofore been widely used by industries such as papermakers and textile mills, who apply the treatments to goods as far-ranging as pet food bags, candy wrappers and carpeting.
Recently, 3M announced that it would stop making many of its well-known Scotchgard® products, after tests revealed that the chemical compound found in such products (PFOS) and in other similar commercially-available products persists in the environment and in the human body for years. The products affected include many Scotchgard® products, such as soil, oil and water repellent products, fire-fighting foams and specialty components for other products. They also include 3M's Scotchban® products and coatings used for oil and grease resistance on paper packaging.
Because PFOS has been found to resist natural processes of decay and can linger in the environment for decades, it falls within the class of chemicals that are notorious for such persistence in the environment, including chlorinated fluorocarbons (CFCs) and polychlorinated biphenyls (PCBs). Government regulations now prohibit the use of either of these chemicals in products, due to their harmful effects on human health and on the environment. CFCs, for example, were found to be causing the destruction of the earth's ozone layer. Other halogenated chemicals of this class cause a wide range of health problems in animals and in people.
While it has been known for some time that PFOS is long-lived, it is only as a result of sophisticated new testing techniques developed in the last few years that the fluorochemicals can be detected in small concentrations in people, as well as in wildlife, water and other areas of the environment. While the studies conducted to date have not demonstrated any hazards to human health from PFOS, it has been shown to be toxic to laboratory animals at high doses. Further, the mere fact that PFOS is a synthetic compound that does not easily degrade raises health concerns. Those concerns, coupled-with the fact that PFOS has been found to accumulate in human and animal tissues, has caused products containing fluorochemicals to be removed from the marketplace and/or discontinued.
U.S. Pat. No. 5,603,996, to Overcash et al. discloses a coated sheet material that includes a porous cellulose substrate sheet material, having a barrier coating thereon which is a blend of a cross-linkable polymer that is resistant to penetration by water moisture when cured and a water-dispersible, film-forming polymer that is resistant to penetration by grease and oil when cured. Disposed on the barrier layer is a release coating, which consists of a fatty acid complex of a metal ion, which is cross-linked to the film-forming polymer in the barrier layer.
Another approach has been to coat a mixture of polyvinyl alcohol and a chrome-fatty acid complex, such as Quilon®, onto a paper substrate. Quilon®, manufactured by the DuPont Company, is a dark, blue-green, chemically reactive, Werner complex in which a C14–C18 fatty acid is coordinated with trivalent chromium in isopropanol solution.
As with the halogen-containing compounds, the use of heavy metals such as chromium, nickel and lead, has raised numerous environmental and health concerns. As such, their use in an oil and grease resistant coating, such as Quilon® or the coating disclosed in U.S. Pat. No. 5,603,996 to Overcash et al, is undesirable, particularly when the coating is one which is to come in direct contact with food products.
Silicone release papers are widely used in the manufacture of layered pressure sensitive adhesive materials or “sandwiches.” The release paper is adhered to an adhesive which has been applied to a face sheet of the subject sandwich. When the sandwich is ready for use, the release paper is stripped from the face paper exposing the adhesive. The adhesive can then be used to secure the face paper. This type of technology has found a wide application in, for example, self-adhering envelopes, labels, postage stamps, stickers and the like.
Typically, the silicone required to impart easy and clean release to paper is applied on a gravure coater off line to a typical fourdrinier wet laid paper machine. The resulting process requires additional handling, process losses and costs, even after the paper has been manufactured. The release paper produced in this manner is also rendered unpulpable for future recycle or reuse. Also, the base paper used for silicone treatment requires a high level of cellulose fiber refining and calendering in order to achieve a very tight sheet pore structure (as measured by air porosity). The subject base paper is typically a supercalendered kraft paper or glassine type product
For some time papers coated with release agents have been known as release papers. The most common material is silicone, which can be applied using either a solvent-based or water-based process. According to Asia Pulp and Paper, there are at least six categories of base materials that can be used with silicone: polyethylene laminated paper; glassine paper; supercalendered-kraft paper; clay coated paper; water resin coated paper; or even a plastic film instead of paper (Asia Pulp and Paper, Volume 30, Number 3, pages 72–77). These papers are then coated with silicone with an aqueous or an organic solvent-based system. At present the solventless approach has largely been driven by legislation on airborne emissions, but nevertheless, the difficulty of recycling the standard silicone treated release papers will continue to remain a challenge. See, e.g., D. Jones in “Silicone Coated Release Liners” (2000 Recycling Symposium, Washington, D.C., USA, Mar. 5–8, 2000, Volume 1, pages 207–208 Atlanta, Ga., USA: TAPPI Press, 2000).
U.S. Pat. No. 5,962,098 discloses the use of a release liner having a flat base substrate which coated on both sides with thermoplastic polyolefin. On one side, the base substrate has a release coating which forms a release force to adhesives. On the other side, the base substrate has a polymer-bound mineral particle layer. This method includes coating a polyolefin onto the paper substrate, which makes it non-repulpable. U.S. Pat. No. 6,210,767 discloses a release liner carrier web including a paper layer, a release layer including a polypropylene layer coated onto the substrate and a silicone release agent coated onto the polypropylene coating layer. The liner further includes a second coating layer made up of an acrylic resin type material for sealing the paper substrate and preventing curling and contamination of a label or printed film adhesively laminated thereon. Again, this patent discloses a plastic film laminated onto the base paper prior to silicone coating. Not only does this make the release paper non-repulpable, it also involves two post-machine processes: laminating and coating. This adds significantly to the cost of the process as well as non-repulpable waste.
U.S. Pat. No. 6,001,473 discloses a release agent made from a starch ester and a plasticizer: “Now it has been found that release coating compositions made from selected starch esters provide good release properties as well as being biodegradable and environmentally friendly making them particularly useful in paper applications where repulpability and recyclability are desired.” Therefore, a repulpable release paper can be produced with the right coating.
Commercial release coatings usually provide two functions to the paper to which they are applied: release and solvent resistance. Since many pressure-sensitive adhesives contain solvents such as toluene, a release coating must have solvent resistant functionality. Without this resistance, the adhesive will migrate into the release paper, inhibiting release. Polymers such as vinyl acetate, vinyl acrylic, acrylic and acrylonitrile exhibit such solvent resistance. The other part of a release coating would be the release component, which may be a silicone or non-silicone organic polymer. Silicone release agents include silicone homopolymers, silicone copolymers, and a blend of the first two. Non-silicone release agents can be either an olefin, a polymer with a long-chain alkyl group, or a fluorine based polymer (Asia Pulp and Paper, Volume 30, number 3, pages 72–77). Usually, both silicone and non-silicone coatings require application after the paper machine with a coater using gravure, blade, rod or air knife technology. Coatings usually need to be high solids, with a high web temperature following to set the release coating. In other cases, some release agents need ultraviolet (UV) or electron-beam (EB) curing after the web dries.
As stated above, the base sheet for the release coating is critical for the release property. Methods involving lamination of a plastic film onto the base paper would render the release paper non-repulpable. Glassine and supercalendered kraft papers are widely used as a release base. Both types of sheets involve high levels of refining on wood pulp fibers prior to papermaking. Refining of wood pulp fibers cuts and collapses the structure of the fibers, causing the fibers to form into a strong, dense structure. The porosity of the resulting sheets is minimal with good sheet smoothness. However, a significant amount of energy is consumed in the process of refining these fibers. The resultant paper web is also difficult to dry in a standard Fourdrinier paper machine due to the density of the sheet This leads to slower machine speeds and increased production costs.
In addition, heavy refining affects the dimensional stability of the release sheet, which could affect runnability of the release sheet. A recent article disclosed that “[t]he impact of runnability on production performance at the customer end dictates the necessity of having a release paper that consistently performs without curling or welting, regardless of the demands of the production environment. Low-density liner (LDL) products combine the surface holdout feature of conventional super-calendered kraft with lower internal-fiber density. The resulting product is lower in basis weight at a standard caliper and is also much less subject to curl after laminating to film or paper facestocks. While there is not a wide array of LDL products currently available, the market seems to hold promise from a growth standpoint The less-demanding die-cutting requirements for the electronic data processing market hold the most promise for LDL products.” Release Papers: New Trends Are Driving the Market, Tardiff, Bob: Plainwell Paper Inc., Paper Film Foil Converter 74, no. 6: 2 PP, June 2000. Therefore, highly refined, high-density release sheets can be susceptible to curl and runnability problems. A lower density release sheet with holdout properties would be preferred in some release applications.
Therefore, the need exists for a formulation which is resistant to oil and grease, and which can be used as a coating or treatment for products for which oil and grease resistance is desired. In particular, the need exists for a formulation which is resistant to oil and grease for products that come into direct contact with food, such as paper packaging, wrappers and containers for food products and the like, but which does not make use of or otherwise contain fluorochemicals.
The need also exists for a moderately dense release sheet with holdout properties that can be coated with a repulpable release coating to provide better release than commercial non-silicone release coatings. In particular, the need exists for a wet end chemistry that maximizes holdout of a low solids release coating that incorporates both a solvent resistant agent and an organic release agent